Nowadays, various disk drives such as magnetic and optical disk drives have become standard equipment of computers, and the reading speed thereof is getting higher and higher. Therefore, the seeking operations of the magnetic or optical read/write heads for desired tracks are required to be as quick and precise as possible.
Please refer to FIG. 1 which schematically shows a carriage structure of a typical optical read/write head. The optical read/write head 10 is carried by a sled 11 and further fine tuned by a lens actuator (not shown), e.g. an electromagnetic actuator, to move along a vertical focusing direction F and/or a horizontal tracking direction T. For a quick seeking operation in the tracking direction, the move of the sled and the lens actuator is based on certain velocity profiles which are established according to the desired move distances (track counts) of the optical read/write head 10. In general, each velocity profile includes an acceleration zone 21, a constant zone 22 and a deceleration zone 23, as shown in FIG. 2. The constant zone 22 is possibly absent due to a relatively short travel.
In order to move the optical read/write head according to a velocity profile shown in FIG. 2, a conventional velocity feedback mechanism shown in FIG. 3 is used to dynamically adjust the outputs of the sled motor 31 and the lens actuator 32. In the velocity feedback mechanism, velocity profiles are provided by a velocity profile generator 30. A velocity sensor 33 is provided to detect a relative velocity of the optical read/write head to an optical disk so as to generate a reference signal S1. The reference signal S1 is feedback, in a reverse phase, to the output end of the velocity profile generator 30 to realize a velocity error Verr. The velocity error Verr is provided for a first and a second compensators 35 and 36 which are electrically connected and output a first and a second control signals S2 and S3 to the sled motor 31 and lens actuator 32, respectively, in order to adjust the velocities of the sled and the lens, and approximate the velocity error Verr to zero. Further, a track counter 37 is used in the velocity feedback mechanism to determine the traveled track number that is provided for the reference of the velocity profile generator 30.
Owing to inertia, the lens movably mounted on the sled will shift to the left side when the sled is driven to move rightwards by the sled motor. Afterwards, the lens driven by the lens actuator is likely to shift to the rightmost end because of the quick response of the lens actuator compared to the sled motor. Consequently, it will take plenty of time to center the lens on the sled to have best performance after a long-distance travel along the tracking direction T for long seeking. On the other hand, during the period the lens is biased, the light intensity realized by the optical read/write head is reduced so as to adversely affect the determination of the moving velocity and direction of the optical read/write head. Such effect may lead to a standstill or an overshoot motion of the optical read/write head, and thus render failure in long seeking.
In order to solve the aforementioned problem, another velocity feedback mechanism, as shown in FIG. 4, is suggested to dynamically adjust the outputs of the sled motor 41 and the lens actuator 42. In this velocity feedback mechanism, the velocity error Verr outputted from the velocity sensor 43 is provided for the first compensator 45 only to modify the output of the sled motor 41. On the other hand, a central error CE in lieu of the velocity error Verr is used by the second compensator 46 to modify the output of the lens actuator 42 in order to center the lens on the sled at any time. Under this circumstance, the second compensator 46 and thus the lens actuator 42 contribute little to the compliance of the velocity profile of the optical read/write head with the required velocity profile. In other words, the sled motor 41 plays the role of carrying the optical read/write head to move according to the velocity profile on its own. When the eccentric rate of the optical disk is great to an extent, and the access speed of data is getting higher and higher, the overshoot problem of the sled motor 41 becomes serious so as to render the system unstable.